When subjected to rapid variations in temperature, the level between the cover-support means of a surface-water drain or manhole and the surrounding ground surface will often vary quite considerably. Since this type of drain is normally located in drive ways, such as roads, such variations in level present a driving hazard and are expensive to rectify.
Such variations in level are mainly caused by the ground freezing during the Autumn and Winter months, the ground, which contains water, expanding when frozen, with subsequent lifting of the ground level, and by partial unfreezing of the ground in the vicinity of the drain or manhole during the Spring months, with reduced firmness of the ground as a result. By way of example, it can be mentioned that when ground which contains a lot of water is subjected to extreme cold over a long period of time, the ground level can rise from 30-40 cm.
Aforementioned local variations in level can occur, among other things, as a result of the ground freezing at different depths at different moments in time, and also because the extent to which the ground unfreezes varies with depth and with distance from a drain or manhole. The following events can take place in the case of a conventional drain or manhole, hereinafter referred to generally as water drain. When the ground freezes, the level of the ground rises, causing the drain cover-support means to be lifted, so that the ground surface remains substantially flat. In warmer weather, the ground in the immediate vicinity of the drain will often unfreeze more rapidly than the surrounding ground, consequently reducing the firmness or supporting power of the ground adjacent the water drain, which may either result in the whole of the drain sinking relative to the surrounding ground, to form a pit, or as is more usual, causing the ground layer immediately surrounding the drain cover to disintegrate and sink to form a crater around the drain cover. The reason why the ground around a water drain will unfreeze more rapidly than ground more distance therefrom is sometimes because of the water from melted snow or ice entering the outside or inside of the drain, thereby heating the same. Air currents in the drain are also liable to heat the same. This is particularly noticeably when steam is used to unfreeze a frozen drain. Sometimes a drain will remain in a lifted position when the level of the surrounding ground has fallen, as a result, among other things, of ground which remains frozen at a deeper level, the raised drain constituting an obstacle and a hazard to traffic.